This disclosure relates to ceramic cores, methods of manufacture thereof and articles manufactured from the same.
Components having complex geometry, such as components having internal passages and voids therein, are difficult to cast using currently available methods. The tooling used for the manufacture of such parts is both expensive and time consuming, often requiring a significant lead-time. This situation is exacerbated by the nature of conventional molds comprising a shell and one or more separately formed ceramic cores. The ceramic cores are prone to shift during casting, leading to low casting tolerances and low casting efficiency (yield). Examples of components having complex geometries that are difficult to cast using currently available methods include hollow airfoils for gas turbine engines, and in particular relatively small, double-walled airfoils. Examples of such airfoils for gas turbine engines include rotor blades and stator vanes of both turbine and compressor sections, or any parts that need internal cooling.
In current methods for casting hollow parts, a ceramic core and shell are produced separately. The ceramic core (for providing the hollow portions of the hollow part) is first manufactured by pouring a slurry that comprises a ceramic into a metal core die. After curing and firing, the slurry is solidified to form the ceramic core. The ceramic core is then encased in wax and a ceramic shell is formed around the wax pattern. The wax that encases the ceramic core is then removed to form a ceramic mold in which a metal part may be cast. These current methods are expensive, have long lead-times, and have the disadvantage of low casting yields due to lack of reliable registration between the core and shell that permits movement of the core relative to the shell during the filling of the ceramic mold with molten metal.
Development time and cost for airfoils are often increased because such components generally require several iterations, sometimes while the part is in production. To meet durability requirements, turbine airfoils are often designed with increased thickness and with increased cooling airflow capability in an attempt to compensate for poor casting tolerance, resulting in decreased engine efficiency and lower engine thrust. Improved methods for casting turbine airfoils will enable propulsion systems with greater range and greater durability, while providing improved airfoil cooling efficiency and greater dimensional stability.
Double wall construction and narrow secondary flow channels in modern airfoils add to the complexity of the already complex ceramic cores used in casting of turbine airfoils. Since the ceramic core identically matches the various internal voids in the airfoil which represent the various cooling channels and features it becomes correspondingly more complex as the cooling circuit increases in complexity.
With reference now to the FIG. 1, an exemplary double wall turbine airfoil 100 comprises a main sidewall 12 that encloses the entire turbine airfoil. As may be seen in the FIG. 1, the main sidewall 12 comprises a leading edge and a trailing edge. Within the main sidewall 12 is a thin internal wall 14. The main sidewall 12 and the thin internal wall 14 together form the double wall. As may be seen, the airfoil comprises a plurality of short channel partitions 13, 15, 17, 19 and 21. The double wall construction is formed between short channel partitions 17, 19 and 21 whose ends are affixed to the main sidewalls. As can be seen in the FIG. 1, there are a plurality of channels 16, 18, 20, 22, 24, 26, 28, 30 and 32 formed between the main sidewall 12, the channel partitions and the thin internal wall 14. The channels permit the flow of a fluid such as air to effect cooling of the airfoil. There are a number of impingement cross-over holes disposed in the partition walls such as the leading edge impingement cross-over holes 2, the mid-circuit double wall impingement cross over holes 4 and 6, and the trailing edge impingement cross-over holes 8 through which air can also flow to effect a cooling of the airfoil.
As may be seen in the FIG. 1, the exemplary double wall airfoil comprises four impingement cavities 22, 24, 26 and 28 in the mid-chord region. The impingement cavities 22, 24, 26 and 28 are formed between the main sidewall 12 and the thin internal wall 14. While the double wall construction of the FIG. 1 provides adequate cooling during the operation of the turbine airfoil, it is difficult to manufacture a ceramic core that comprises all features of the cooling passages 22, 24, 26 and 28 during a single operation.
The double wall construction is therefore difficult to manufacture because the core die cannot be used to form a complete integral ceramic core. Instead, the ceramic core is manufactured as multiple separate pieces and then assembled into the complete integral ceramic core. This method of manufacture is therefore a time consuming and low yielding process.
It is therefore desirable to have an improved process that accurately and rapidly produces the complete integral ceramic core for double wall airfoil casting without having to manufacture multiple separate pieces and then assembling them.